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Piero

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Everything posted by Piero

  1. Sidgwick's Amateur astronomer's book is excellent in my opinion. Very inspirational and rich of insights. A keeper.
  2. Thanks. Burnham cites that book quite often. That's how I became aware of it.
  3. By 'tipped' do you mean off-axis towards the primary mirror? 1. A primary mirror axial misalignment will cause coma on axis (coma due to misalignment). 2. A focuser axial misalignment will cause the stars to focus at different points across the focal plane. 3. A secondary mirror (severe) misalignment will cause unequal field illumination. It's rather obvious that only the first one degrades on-axis, therefore 2 and 3 reveal nothing on a star testing as this is conducted on-axis. Of the 3, the last one is the less critical for visual astronomy.
  4. If collimation of a, let's say, f5 Newtonian telescope without coma corrector is checked with a star test, the star must perfectly be on axis, otherwise the coma-dependent misalignment is also visible. This cannot be really distinguished by optical misalignment. So, again, to me at least star testing is not the right way to check collimation as it is too sensitive, unless the seeing is very good, the high power eyepiece is good, coma corrector is used, and the mount tracks automatically. Even so, this test doesn't tell you anything about secondary and focuser alignments.
  5. Not sure what some members above meant by star testing in this thread. Do they mean "checking" or "collimating" with a star test? In my opinion, I wouldn't suggest the latter, particularly with manual driven mounts. Regarding the former, the procedure can be rather complex as it is easy to get errors due to other factors which don't have anything to do with misalignment. Of course, if one knows how to star test, the feedback given by this tool is incredibly useful in order to understand what does not work properly in a telescope. Regarding the importance of collimation, well
  6. I tend to stand while observing as that's my preferred way. This with both my dobson and refractors. The refractor mounts have an extensible column which I love as it avoids contortionism or "yoga postures".
  7. Last Thursday night the seeing was great with very good transparency. Observing Aristarchus crater and plateau from 160x to 630x was amazing. In particular, I was impressed by the kind of "river" approaching the crater to the left and how the light played fantastic shades inside. From Wikipedia: Aristarchus plateau (NASA).
  8. Found these books in the second hand market.
  9. I observed Copernicus past night with my dobson up to 533x without any sign of image degradation. It was wonderful!
  10. Last night I had a chance to try the telescope following the last work on the mirror cell. The sky conditions were clear, windless, and rather stable temperature between 8.20pm and 9.20pm. After that the temperature started dropping. The telescope was left outside with the fan on since 6.30pm. The light shroud was also fixed in order to reduce internal and nearby turbulence. Both telescope focuser and primary mirror axial alignments were adjusted with my 2" Glatter laser 650nm. Several stars were used for star testing: Betelgeuse, Procion, Rigel, Pollux, and Aldebaran. This was
  11. I thought you already used a RACI finder with your dobson. Nice finder! At the moment I have an Antares VS60 with no illuminated field.
  12. I worked another bit last night and this morning: replaced temporary nylon pins + 2mm pad on top with a stack of 2 x 5mm thick pads (20mm diam). This raised the mirror 1mm. the milk cartoon structure to maintain the triangles in position without affecting their movement was moved underneath marked the position of the sling (COG) on four points. These are about equidistant. cut velcro strips in two segments and placed them under and above the COG where the sling will pass. The external border of the velcro strip (which is slightly flat) was placed towards the slin
  13. I ordered a set of felt pads of different sizes and 5mm thick. After taking all the measurements, I need to stack two of these in order to replace the pins. Will do the work tonight or tomorrow morning. What's the diameter of your pads installed by Randy? 1inch, 20mm, or more?
  14. Hey Gerry, Discard the z function (green) as not necessary here. What we care here is the part of the functions where x is from 0 to pi/2 rad (90 deg). This is the movement of the telescope regarding altitude. Basically, depending at 0 deg telescope altitude the weight force is all on the sling (see cos function) and this decreases as the altitude grows to 90 deg (pi/2). Follow the sin function for studying how the weight force works on the triangles. Note that the intersection is not at 0.5 y. Source: Wikipedia.
  15. Here we go You don't need the actual W vector for this, so it can be assured to be the unit vector. What really matters are the sine and cosine functions at different angles alpha. The alpha angle is 90 - telescope_altitude. Sine and cosine receive angles in radians, so you need to covert: 1deg = pi/180 = 0.01745 rad.
  16. Thank you, Gerry. Very helpful. Placing a pad on top of the pin is not a final solution, but only a quick way to test the idea. I certainly need to consider either taller pins or taller pads, once I get the the measure of the exact height. If this is exact there is no friction caused by pins or pads.
  17. Gentlemen, you are too kind! I haven't come up with new products or design, but just put some information together and been trying to analyse / address some issues which are often ignored. Hopefully, this thread will become a useful resource for those of us having to deal with these issues or want to improve their telescope performance. That would be great! I feel the thread title should change. Currently, it is not really informative and this might affect searches. Probably something like "Lukehurst-Nichol classic dobsonian mods" would be more appropriate? If happ
  18. It's late here Gerry! What's your humblest suggestion? I'm curious!
  19. I don't think it is linear. I suspect it is sin/cos, but haven't thought about this properly.
  20. Sling and triangle supports play orthogonal (=perpendicular) forces AND they should do so, otherwise the result is astigmatism (at the least). Triangle support is 0 at 0 deg altitude and 1 at 90 deg altitude. Sling support is 1 at 0 deg altitude and 0 at 90 deg altitude.
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